linux-kernel - Re: Definition of BUG on x86

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Date:	Thu, 19 Feb 2009 17:11:23 +0100
From:	Petr Tesarik <ptesarik@...e.cz>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Definition of BUG on x86

Ingo Molnar píše v Čt 19. 02. 2009 v 16:35 +0100:
>[...]
> * Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> 
> > Ingo Molnar wrote:
> >> * Petr Tesarik <ptesarik@...e.cz> wrote:
> >>
> >>   
> >>> Ingo Molnar píše v Čt 19. 02. 2009 v 13:47 +0100:
> >>>     
> >>>> so GCC should be fixed and improved here, on several levels.
> >>>>       
> >>> Agree.
> >>>
> >>> But it takes some time, even if we start pushing right now. What's 
> >>> your suggestion for the meantime? Keep the dummy jmp? And in case 
> >>> anybody is concerned about saving every byte in the text section, 
> >>> they can apply my dirty patch?
> >>>
> >>> Actually, this doesn't sound too bad.
> >>>     
> >>
> >> yeah. Please forward the problem to the appropriate GCC list in any 
> >> case.
> >>
> >>   
> >
> > I think the official answer for this case is to use __builtin_trap.  But:
> >
> > -- Built-in Function: void __builtin_trap (void)
> >     This function causes the program to exit abnormally.  GCC
> >     implements this function by using a target-dependent mechanism
> >     (such as intentionally executing an illegal instruction) or by
> >     calling `abort'.  ***The mechanism used may vary from release to
> >     release so you should not rely on any particular implementation.***
> >
> > which in principle is hard for us to make use of.  In practice I think  
> > it has always been ud2a on x86.
> 
> could we just do:
> 
> 	__builtin_trap();
> 	for (;;);

I'm afraid that's not the point of the exercise. I'm trying to trim
BUG() to two bytes, while still making sure that the Illegal Opcode
exception is generated at the exact code point, so we can track it down
using the info in __bug_table. If __builtin_trap() ever translates to
anything else than ud2a in the above code snippet, there will be no BUG
reported. Instead, the CPU that encountered the BUG() will burn CPU
cycles forever without any apparent reason.

NAK,
Petr Tesarik

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